The invention relates to a control motor comprising an integrated direct current motor and speed control circuit and, more particularly to a control motor which is suitable for use as a blower motor provided in a vehicle air conditioner and an electrically-driven fan motor for cooling a radiator.
In the vehicle air conditioner a rotational speed of the blower motor is controlled in order to regulate air flow blown in a vehicle compartment.
In some of the speed control circuits of this kind, the rotational speed of the blower motor is controlled linearly by a power transistor. The advantage of linear control is that little noise is generated; the disadvantage to linear control, however, is that heat is generated in the power transistor. The Japanese Patent Application No. 235114 of 1988 discloses an intake unit 110, in which a switching element 113 installing a heat sink 112 is positioned near a scroll 111 of the intake unit 110 to radiate the heat, as illustrated in FIGS. 29 and 30.
The Japanese Patent Application No. 235114 of 1988 also discloses the intake unit 110, in which a voltage is controlled by a chopping performed by a pulse width modulation control (PWM control) to control generation of heat in the speed control circuit and a power MOSFET generating only a little heat is used as the switching element. In such a motor as disclosed in the Japanese Patent Application No. 235114 of 1988, there is a relatively little heat generated in the speed control circuit and radiating the heat is easy, therefore the speed control circuit is placed in a motor bracket to be formed integrally with the direct current motor, leading to an easy installation of the direct current motor. This control motor comprised of the integrated direct current motor and the speed control circuit is referred to as a smart motor or an intelligent motor.
The speed control performed by the chopping has a disadvantage in that radio frequency noise is generated by the speed control circuit. In a motor loaded into the vehicle a power source is a battery and the voltage is low. Further, in a large motor, a high current of more than 30A must be switched. In this switching, a float inductance and a float capacitance of the speed control circuit produce a voltage and a current oscillation of a far higher frequency than the switching frequency, generating radio frequency noise. The radio frequency noise may conduct to other electronic apparatuses connected with the same battery with the radio via the power line, producing harmful influences such as faulty operation of a microcomputer. Further, the noise is radiated directly as an electromagnetic wave, exerting a harmful influence on electronic apparatuses provided in a periphery of the radio. Also, the noise intrudes from an antenna of a radio loaded in a vehicle, preventing receipt of the radio frequency from a broadcasting station. In a conventional control motor having a speed control function and using a high current, no consideration was given to the above radio noise.